In an electronic display, a material, called the display medium, changes its optical state, e.g. reflectivity, transmission or light emission, in response to an electrical signal such as a voltage or an injected current. Particularly widely used are liquid crystal displays (LCDs), which change either the polarization of light or the reflectivity for light under the influence of voltage. Typically the display architecture comprises a transparent front substrate coated with a transparent conductive electrode, e.g. Indium Tin Oxide (ITO), which may be patterned or unpatterned (the front plane), a back plane comprising electrodes which may optionally be transparent and an optional substrate which may also optionally be transparent, and the display medium place between the front and back plane. The control of the optical state of the display results from the application of a voltage between the front and back plane electrodes which creates an electric field across the display medium or injects a current into the medium. The display may be divided into segments or, for more complicated information, into so-called pixels organised into a matrix of rows and columns.
Some display media display image retention without being actively addressed, i.e. once an image is written to a display based on such a medium, no further power is needed to maintain the image, at least for a significant amount of time. Such displays are commonly called bi-stable, even though many such displays are not strictly speaking truly bi-stable (having two stable states of equal energy), and this terminology will be applied herein as well.
Two types of bi-stable displays that have generated significant interest for “electronic paper” applications are electrochromic displays, such as reported and provided by NTera Inc. (www.ntera.com), and electrophoretic displays such as are reported and provided by E Ink Inc. (www.eink.com) and Sipix (www.sipix.com). These displays show optical properties that in many ways are similar to ink on paper. However, these displays generally require an active matrix structure (an active electronic circuit controlling the electronic and optical state for each pixel) in order to display detailed information, as they are not addressable by passive matrix techniques and/or the response times are too slow for passive matrix line by line addressing.
Active matrix circuits may be constructed on the basis of thin film transistors (TFTs), which are three terminal devices, or two terminal devices such as rectifying diodes and MIM diodes. TFTs have become the standard technique for LCDs. In a TFT display, patterning is all done on the back plane of the display, with row and column lines connected to gate and source electrodes; the front plane electrode is typically unpatterned throughout the active area of the display. This has the advantage of reducing the requirements for alignment of front and back planes during assembly, which is especially important for displays which are made using printing technology and/or flexible substrates, e.g. using organic semiconductors for the TFTs (see, for example, www.plasticlogic.com). On the other hand, TFTs require very fine patterning steps and excellent semiconductor characteristics, especially charge carrier mobility, to perform adequately, and both issues become difficult when using printing technology and organic semiconductors.
A number of diode-based active matrix back plane structures have been disclosed in the past and some have also been used commercially. Two structures are the diode ring and the back-to-back diode configuration, as seen in FIG. 1.
The display medium is in series with the diode circuit; the diode circuit and the display medium are placed between the row and column electrodes.
Another diode circuit design is called the two-diode switch, and the circuit is illustrated in FIG. 2. This circuit configuration requires both a patterned front plane electrode structure and two select lines for each row of pixels.
Patent application WO 2004/066410 discloses diode based circuits for active matrix back planes using single diode and back-to-back configurations, where the diodes are thin film diodes made using organic semiconductors (FIG. 3). Two potential advantages of organic diodes over organic TFTs are less stringent requirements on charge carrier mobility and less stringent requirements on resolution due to the vertical instead of horizontal structure of the charge conducting layer. These two advantages can be especially significant for cost-efficient, high-throughput printing based manufacturing of active matrix back planes, as the resolution achievable with standard printing methods is limited. As in FIGS. 1 and 2, a patterned front plane electrode is used.
Metal-insulator-metal diode (M-I-M) diodes differ from rectifying diodes in that the current-voltage curves are symmetrical. MIM diodes have also been used to form active matrix back planes for displays; and example structure is shown in FIG. 4. Note that also here the display medium (here a liquid crystal) is in series with the diode element in between row and column electrodes, i.e. the front plane electrode must be structured.
Active matrix back planes based on solution processible MIM diodes have been disclosed in patent application disclosure WO 2004/051750 and in U.S. Pat. No. 6,380,922. In the former no description of a circuit or reduction to practice is given. In the latter, printed MIM diodes based on inorganic metal oxides are claimed for use in active matrix displays, preferably electrophoretic displays. An unpatterned front plane electrode is used in the proposed circuit, which can drive a single pixel, but no disclosure is made how an entire matrix of rows and columns could effectively be driven.
U.S. Pat. No. 6,980,196 discloses an electronic display where the display medium is particle based, e.g. electrophoretic, and controlled by an active matrix which may comprise printable nonlinear diode elements, the intention being the fabrication of a printable active matrix display. The display medium is placed in series with the nonlinear device between row and column electrodes, i.e. the disclosure requires patterned front plane electrodes. However, as mentioned above, having structured elements on both the front and back plane create additional alignment difficulty, especially if printing and lamination processes are used for display fabrication and assembly. Furthermore, some display media, e.g. electrophoretic and polymer-dispersed liquid crystal media, are frequently commercially available primarily as front plane laminates with the active display medium already laminated onto an unpatterned front plane.